Wordpress - How do I get the category URL from get_the_category?

Use:

get_category_link( $category_id );

See:

https://codex.wordpress.org/Function_Reference/get_category_link

In your specific case:

<?php
global $post;
$categories = get_the_category();

    foreach ($categories as $category) :

       $exclude = get_the_ID();
       $posts = get_posts('posts_per_page=4&category='. $category->term_id);

        foreach($posts as $post) :
         if( $exclude != get_the_ID() ) { ?>

                <a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>" class="post c-1"> Link to actual post</a>

    <?php } endforeach; ?>

<a href="<?php echo esc_url( get_category_link( $category->term_id ) ); ?>" title="View all" class="btn border"><i class="i-right-double-arrow"></i> View all <?php echo $category->name; ?></a>
<?php  endforeach; wp_reset_postdata(); ?>